The kortholt is essentially a folded cornamuse.
The bore doubles back on itself to give an instrument with a
"musical" length twice its actual length. Apart from the obvious
advantage of compactness there is a further benefit. This is that
some additional fingerholes can be drilled for the "upward" bore to
provide some extra notes below the normal range. The fingering
becomes slightly tricky requiring the use of all ten digits plus
the joints of the two index fingers. However, a tenor instrument
can have its range extended down from "c" to "F" and a bass from
"F" to "BBflat".